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MG MGB Technical - Engine Mount Height Difference, Rubber Bumper

I was getting ready to put new engine mounts in my 77 roadster today, when I noticed that the mounts I ordered are not the same height (thickness). They are almost surly made by different manufacturers, and there is no indication of who or where that might be.

One of them is 1-13/32 inch from metal to metal, and the other is 1-18/32. So, one is 5/32 inch taller than the other. The taller one looks like better quality to me, and is painted flat black. The shorter is painted with gloss black. The metal plates are the same thickness, so the difference is in the rubber.

They each have two part numbers, and they are the same. In larger print is 413-060, and in smaller print is BHH1621.

Am I correct that they are supposed to be the same? I am confident that the supplier will make it right, but does anyone know if one height would be preferred over the other?

Thanks,
Charley
C R Huff

Charley-
I just measured a pile of dead ones, probably OE. Across the outside of the metal, they are all 1.5+ - 1.6, except one obviously squashed one, which is still more than 1.4. Or maybe we just found out why the cars have different amounts of shim plates! They are supposed to be the same.

FRM
FR Millmore

FRM,

Thanks for taking the time to measure your pile-o-mounts. Seeing your decimals reminded me that my vernier calipers, which I abandoned as my eyes became less sharp, is here at the house. I did a remeasure and got 1.605 and 1.455 inches. The difference, 0.15, is about 5/32, but the total height is a bit more than I got with the tape measure. It looks like I need to get another of the taller ones.

Regarding shims, my older chrome bumper GT with the rectangular mounts did have one shim, but this rubber bumper model with the round mounts didn’t have any. However, it did have what could be shims on the underside of the frame mounting point. I assumed these were to stiffen the mount points, but maybe they are shims that someone was storing for future use. Do you know if it is normal to have a backing plate hidden on the underside of the mounting point on the frame?

Charley
C R Huff

The round mounts always have a square plate on the back of the chassis bracket with an offset hole. This is a location plate to ensure the engine is sitting at the right height between the mounts. It goes on the stud with the hole in the lower position, i.e. the bulk of the plate going up inside the mount. If you can't get it on that way round then it means that the engine is too high, which could be because you have too many optional shims on the top-side of the chassis mounts.

Conversely you can have it too low as well, the studs mustn't be at the bottom of the slots in the chassis plates or when the nuts are tightened the mounts will be in tension and shear instead of compression. One or more shims is added as required to lift the studs above the bottom of the slots, but not so much you can't get the location plates on.

The shims are required because the engine is effectively sitting in a Vee and there is a certain amount of manufacturing tolerance in the distance between the two halves of the Vee - the bigger the distance the lower the engine sits needing shims as required, at the closest tolerance you may not need any shims. A similar thing applied to the square mounts to enable the round holes to be aligned without the rubber mounts being under tension and shear.
Paul Hunt

Thanks Paul. I saved your instructions and I'll check the positioning accordingly when I reassemble it.

Charley
C R Huff

Paul-
All good, but:
" the studs mustn't be at the bottom of the slots in the chassis plates"
>>> Gotta disagree with this part; the studs in fact go all the way to the bottom of the slots, and the offset of the square plate is such that it can only be installed with the studs in that position.

This will not put the mounts in tension, and in fact if the slots were deeper, the mounts would go into more & more compression. In fact, since the square plates can only be fitted with the studs all the way down, they frequently get discarded; and, it is just these cars that show up for mount replacement, since the mount is in tension if the studs are not bottomed. When missing the square plates, I just tap the studs to the bottom and use big round washers under the nut/spring washer.
If the mounts are still in tension then the round shim plates need to be fitted or increased.

FRM

From my post of several years ago:
You will note that the square washer has an offset hole. The mount stud goes all the way at the bottom of the slot on the frame (tap it down with some weight on the mount), and the square washer offset is to keep it there so it can't move upward under any (loose) condition. Put the square piece in (yes, under the bracket - sorry)so that the widest part is uppermost, it just fits up against the top edge of the cavity. then figure out how to get the lockwasher and nut on!! Run a die over the threads before starting this so the nut is absolutely free-running - the threads are usually bunged up. Use anti-seize on the stud. It helps a bit if you can get the underside of the frame bracket clean (Brakleen or lacquer thinner) and use something sticky to hold the square washer up while you deal with the nut and lockwasher.
A further tip: Have you replaced the brackets on the engine? They commonly are broken, and if not near new are cracked if not broken. Look very carefully at the 90deg bend at the front edge, and around the lower bolthole to the block. You DO NOT want to do this again soon! To greatly increase lifetime, Put 1/8" of spacers between the block and the lower bracket, and a heavy washer under the lower bolt head. You will have to grind this to fit securely in the channel at that point, and you need a slightly longer bolt. Be sure that the longer bolt will tighten against the stack of spacers and bracket without the lockwasher, then fit the lockwasher. If this bolt is bottomed in the hole the bracket WILL break, and the threads WILL be damaged when you remove it. This procedure prestresses the mount bracket and the rubber mount in a favorable direction and increases the life indefinitely. I've never replaced one I did like this, but have done dozens done as original.
FR Millmore

If you need shims to align it properly, use your old set of mounts. The shims that you order are nothing more than the metal plate minus the rubber. A chisel or even a good putty knife will separate them from the rubber. Clean, sand and paint them and you have shims at no charge and no wait for delivery.
Rick Penland

The supplier is supposed to be sending me another mount at no charge, and he said not to bother sending the short one back. So, if I need shims, I have one more mount to cannibalize. Thanks for the reminder.

I saved the notes from both Paul and FRM. When the new mount arrives, I'll check the fit and try to get them in place in a stress-relieved manner.

Charley
C R Huff

Charley-
When all is done and you have it right, the top mount plate will be exactly parallel to the bottom, and the sides of the rubber will be exactly perpendicular to the plates all the way round.

FRM
FR Millmore

FRM,

Thanks. That is what I was thinking of as an indication of it being correct.

Charley
C R Huff

We had this discussion before, FRM. I still believe that if the stud is resting on the bottom of the slot, and especially if there is any clearance between the rubber mount and the chassis mount, then when the nut is tightened it will stretch the rubber which will then be in tension and shear. But I guess we'll have to agree to disagree on that.
P Hunt

Paul-
Ok, but how can there be clearance between the rubber mount and the chassis mount if it's holding up the engine?
And, it is not possible to correctly install the offset square plate flat against the underside of the frame mount if the stud is not at the bottom of the slot.

Gotta go
FRM
FR Millmore

I'll try to pay close attention when I do mine. Maybe I can settle it, or maybe I will come up with a third opinion so that all three of us can agree to disagree!

I am just hoping, when my replacement mount shows up, that it is a mate to the taller of the two that I already have.

Charley
C R Huff

"Ok, but how can there be clearance between the rubber mount and the chassis mount if it's holding up the engine?"

When the weight of the engine is being taken by the stud sitting at the end of the slot, and not by the face of the mount on the face of the chassis bracket. That *has* to be more likely than not, given the tolerance in the construction of the chassis rails if not the rubber mounts, which is why shims have to be added as required i.e. to pad out the gap between the chassis mounts. Thinner mounts, like Charley seems to have had, make it even more likely. It's the only reason shims were even listed in the parts catalogue, which means they must be needed in some cases at the very least, and without them the weight of the engine would be taken on the studs and not by the rubber, and the rubber would be in tension an shear when the nuts were tightened.
P Hunt

Paul-
I'd have to agree, but it does seem like a gross error, sort of like hanging the car upside down! If you put the nut on and tighten, it will tension the mount, putting it in tensile shear as you say; but, it will NOT satisfy my criterion of the mount winding up square and parallel. I stand by my assertion that in the end, with correct shims, the stud goes at the bottom of the slot. I routinely tap the mount down with some weight on it. This is the only position in which the offset square plate will fit, and correct shimming will satisfy the "square & parallel" requirement.

FRM
FR Millmore

This thread was discussed between 22/11/2009 and 30/11/2009

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